In offset lithographic printing, each printing stage includes a plate cylinder, to which printing plates are tightly fastened around its circumference. The plate cylinder is equipped with inking and dampening mechanisms. The plate includes both image and non-image areas which are substantially coplanar, the image portions being hydrophobic and the non-image areas being hydrophilic. The dampening system applies an aqueous solution to the non-image areas and the inking system applies a greasy ink to the image areas. The plate cylinder transfers its image to an intermediate blanket cylinder which has a specially composed smooth rubber blanket surface. Printing stock in either sheet or webbed form is fed against the blanket cylinder by an impression cylinder and the ink (and dampening solution), is transferred to the printing stock thus completing the printing operation.
In applicator roll assemblies used with conventional lithographic printing cylinders, a pick-up roll is partially immersed in a trough containing a continuous supply of liquid. The liquid may be water, ink or a coating composition. The surface of the pick-up roll "picks up" a relatively thick coating of the liquid and rotates it into contact with a metering roll which controls or meters the thickness of the coating which is to remain on the surface of the pick-up roll. Excess liquid is returned to the trough. Further rotation of the pick-up roll brings it into pressure contact with an applicator roll whereby the applicator roll obtains a coating of the liquid from the pick-up roll. Finally, the applicator roll rotates into pressure contact with the plate cylinder (or in some instances the blanket cylinder) which is coated with the liquid by the pressure/rolling action of the applicator roll.
As the applicator roll moves about the outer periphery of the plate cylinder, it comes into contact with a plate clamp aperture where plates are secured to the outer periphery of the plate cylinder. Unless the applicator roll/plate cylinder contact pressure is very closely controlled, the clamp aperture will often cause the applicator role to slightly move away from the periphery of the plate cylinder as the plate clamp aperture passes beneath it. This can create an interruption in the application of the liquid to the plate with resulting nonuniformities in the printed product.
Accordingly, it is an object of this invention to provide an improved liquid transfer assembly for offset lithographic printing apparatus.
It is another object of this invention to provide an improved liquid transfer assembly for offset lithographic printing apparatus wherein uniform layers of applied liquid result from the action of the transfer assembly.